Many processes and catalysts are known for the preparation of olefin polymers. Ziegler-Natta (ZN) catalyst compositions and chromium oxide compounds have, for example, been found to be useful in the preparation of polyolefins. Further, single-site catalysts (e.g. metallocenes) have been used and have been found to afford polymer properties not easily available by using ZN catalysts.
The first catalysts to be developed were homogeneous, i.e. they were used in solution in the polymerisation reaction. Due to the many drawbacks of homogeneous solution systems, several different approaches have been used to try to overcome the problems of the solution catalyst systems. Nowadays the most commonly used catalyst systems comprise heterogeneous catalysts, wherein catalyst components are supported on an external carrier. The carriers used have a porous structure in order to facilitate catalyst impregnation into the support. Carrier materials are typically polymeric or inorganic supports, most typically silica, alumina or magnesium dichloride based materials.
However, even the catalysts supported on external carriers have their drawbacks. The main drawbacks concern the inhomogeneity of the resulting catalyst. From the point of view of catalyst and polymer properties, one of the most important drawbacks of this kind of heterogeneous catalyst system is that it is difficult or even impossible to get solid catalyst particles, wherein the catalyst components are evenly distributed through the catalyst support. Furthermore, the inhomogeneity problem also exists between catalyst particles, i.e. intra and inter particle inhomogeneity problems occur. This is significant as the morphology of the support or carrier material has an essential effect on the catalyst and, due to the replica effect, also on the polymer morphology. Thus, any variations in catalyst distribution and morphology lead to differences in catalytic behavior in polymerisations and result in inconsistencies in the polymer product. In addition, use of an external support involves additional costs, the quality of the support must be carefully controlled, and still sometimes carrier residues might cause some problems.
A catalyst providing the advantages of both homogenous and heterogeneous catalysts is clearly desirable. Whilst it is generally believed that it is very difficult to obtain good polymer particle morphology using an olefin catalyst, which is not supported on an external support, the present Applicant previously found that polymers having a desirable particle morphology may be obtained using such a catalyst, which is in the form of solid particles (but in which an external carrier or support is absent). WO03/000754, WO03/000757, WO 03/051934 and WO03/106510, the contents of which are incorporated herein by reference, describe such catalysts and processes for producing them. The preparation of this kind of catalyst is based on a liquid/liquid emulsion system comprising at least two phases, from which the catalyst particles forming the dispersed phase of the emulsion, are solidified.
Solid catalysts are commonly prepared by using batch processes. However, the use of batch reactors causes problems in the resulting catalysts due to variations in the concentration of different components and in the physical conditions in the reactors with time. In addition there are inconsistencies between the different batches prepared. Moreover, in a batch process, the reactor volumes have to be large if catalysts are to be prepared on a commercial scale.
In the production of solid catalyst supported on an external carrier by a batch process there are also some additional problems. One solution for solving some of these problems during the preparation of Ziegler-Natta type catalysts supported on an external carrier is disclosed in WO02/48208. In the disclosed process the titanium compound is fed continuously into a vessel containing a suspension of a solid comprising a magnesium halide and further discharging liquid from the vessel. As a result a typical solid Ziegler-Natta type catalyst supported on an external carrier, which is especially used in propylene polymerization, is obtained. All temperatures and mixing conditions etc. have to be carefully selected in order to get the desired product. The process of this publication is, however, still far from a continuous process since it only describes a continuous method by which only one part of one step of the preparation of MgCl2 based ZN catalyst can be carried out.
Also the solid catalysts obtained by the liquid/liquid emulsion technology referred to above have been prepared by a batch process comprising the following process steps:    (1) preparing a liquid/liquid emulsion comprising at least two phases, wherein a solution of catalyst component(s) in a solvent forms the dispersed phase in the form of droplets and a liquid medium immiscible therewith forms the continuous phase;    (2) solidifying said droplets from said dispersed phase, and optionally    (3) isolating said catalyst,
Each of the steps (1)-(3) is described to be carried out in batch wise manner. However, the batch-wise processes disclosed in the above references cause additional problems in the above process and furthermore in the produced catalyst as well. Firstly, the process requires the transfer of the emulsion between two reactors. Because the stability of the emulsion is very limited, such transfers and any possible storage needed in between transfers will result in loss of material quality and amount. Further, the solidification is effected by changing the emulsion state by physical or chemical actions, i.e. by adding different chemicals or changing the physical conditions of the emulsion. In a batch-wise process, during the solidification, temperature and solvent concentration vary with time and thus temporal differences occur meaning that individual particles are exposed to different environments, which again leads to catalyst with undesired non-uniform particles and having uneven chemical composition distribution within the particles. Hence the end products obtained using such catalysts are of decreased quality. Also the hold-up of different reagents needed in the process is much higher in a batch process than in a continuous or semi-continuous process. In addition to these specific problems, drawbacks of common batch-wise processes relating e.g. to high reactor volumes in commercial production and common non-uniform products exist.